Chocolate may be the most sought-after treat among trick-or-treaters on Halloween, with little hands grasping for all of the milk- and dark-chocolate morsels they can collect, but the details of its taste and aroma profiles have long eluded scientists.

And new science is revealing why cocoa's potent sensual properties have been so difficult to pin down. A recent analysis found that the individual aroma molecules in roasted cacao beans (the primary ingredient of chocolate) can smell of everything from cooked cabbage to human sweat to raw beef fat. Together, more than 600 of these flavor compounds melt together in just the right combination to yield the taste and scent of what we all call chocolate, according to Peter Schieberle, a food chemist at Munich Technical University and director of the German Research Center for Food Chemistry, who presented the data at this year's meeting of the American Chemical Society in Denver.

Most of the molecules that comprise a food's aroma are volatile, which means they transform into gases easily at room temperature. These volatile compounds are inhaled along with the air we breathe, bringing them into contact with the 900-plus odorant receptors in the upper half of the nostril. In the early 1990s scientists Linda Buck and Richard Axel began the work that would show each odorant receptor recognized one particular compound and was linked to a specific olfactory neuron in the nostril. As a volatile aroma compound latches onto an odorant receptor, it triggers the firing of the olfactory neuron (Buck and Axel won the 2004 Nobel Prize in Physiology or Medicine for their discovery). Complex aromas form when multiple volatile compounds trigger their respective olfactory neurons at the same time. The brain identifies flavor by measuring how frequently the different neurons fire.

"By the time you put four chemicals together, your brain can no longer separate them into components. It forms a new, unified perception that you can't recognize as any of those individual aromas," says Gary Reineccius, a food scientist at the University of Minnesota.

Processed foods such as chocolate, beer and tea contain thousands of aroma compounds. This multiplicity of molecules creates a mosaic of odor in the brain as each individual molecule contributes a hint of scent to the final flavor. Just as our brains can often assemble a whole picture from seeing just a sketch of an image, Schieberle and colleagues found that humans can recognize chocolate aroma using only 25 of its 600-plus volatile compounds. Of these, many are also found in much less appetizing items, including cooked cabbage, raw beef fat and human sweat, which are in turn also composed of many different volatile compounds.

Even so, not one of these 25 key compounds can be pegged as a "chocolate" aroma. "The mixture smells completely different from the individual constituents," Schieberle says. "At the moment, there is no way to predict how the final mixture will smell."

Schieberle calls the study of individual aroma and flavor molecules "sensomics," which sifts through the countless potential aroma compounds for those molecules of particular importance to human taste and smell. Schieberle's work has identified which aroma compounds from roasted cacao beans could bind to odor receptors in humans. None of them, it turned out, smell anything at all like the sweet, rich scent we identify as chocolate.

To figure out exactly which molecules contributed to chocolate aroma, Schieberle and colleagues had to pick apart chocolate aroma one molecule at a time. First, the researchers identified those volatile compounds that would react with human odor receptors and were present at high enough levels to register in the brain, which yielded 25 different molecules. These molecules included 2- and 3-methylbutanoic acids (both produce a sweaty, rancid odor), dimethyl trisulfide (cooked cabbage) and 2-ethyl-3,5-dimethylpyrazine (potato chips). Then, they blended these rather un-chocolatey aroma molecules in different combinations and asked human study subjects to smell them. The blend that contained all of the 25 volatile aroma molecules could reliably fool the nose and brain into thinking it had smelled chocolate.

These 25 compounds are what Schieberle refers to as chocolate's chemical signature—those volatile compounds in chocolate that trigger human olfactory nerves in just the right combination "causing a signal in the brain to say 'this is chocolate,'" Schieberle says.

What we think of as "chocolate" smell is due in large part to the way in which the food is made—a process that includes both fermentation and roasting. Foods that are processed by fermentation, roasting or grilling such as wine, coffee and steak, respectively, generally contain the most aroma molecules. It is this process's conversion of otherwise odorless compounds into volatile aroma-bearing ones that helps explain this type of food's popularity. Natural, raw foods like fruits and vegetables also have an appealing aroma and taste, although their flavor profile is much simpler and usually dominated by one or two major molecules.

"That chemical really creates that flavor, and everything else kind of smoothes it and makes it pleasant," Reineccius says of these less complex foods. The combination of volatile aroma compounds as well as the sugars and salts that we taste during chewing combine to create flavor. "Some of our simpler flavors are strawberry and raspberry because they're just what nature happened to provide to keep itself living." The replication of these flavors by food chemists has previously been a process of trial and error.

The goal of his work, Schieberle says, is not to develop artificial chocolate flavorings. Rather, his goal is to find ways to tweak the cacao bean fermentation and roasting process to develop even better tasting chocolates. A recent discovery in his lab, made earlier this year, has taken a small step in this direction. Cacao beans processed in the so-called Dutch style, which adds alkali salt during roasting, have a milder, more pleasant flavor. After deconstructing the molecular makeup of this form of chocolate, the researchers knew that it contained molecules that had a pleasant "mouthfeel." And by adding a tiny bit of glucose to the cacao beans during the Dutch roasting process, Schieberle and colleagues, did not increase the sweetness of the final product, but instead created a more velvety mouthfeel in the final chocolate.

Better understanding chocolate's alluring aroma can also help with tasting technique. Let the chocolate dissolve on your tongue, Schieberle says, so that you can taste the full array of flavor compounds. As the chocolate melts in your mouth and you exhale, some of the volatile molecules will once again pass over your odor receptors, letting you get another whiff before the chocolate melts away.

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